Monday, 19 February 2018

40m Direct Conversion Receiver - Yes, I'm doing another one!

OK, so I saw a blog post by someone who suggested they were getting a batch of boards done at a great price. I won't name them because their bargain was way too expensive once freight was included. But by the time I discovered that it was too late, I'd made the mental commitment to one final, best I could build version.

Since then I've sat on my hands for almost a year. But this weekend I needed a break from the maths of loop filters and modifying 10GHz equipment. I wanted something I could do without much thought.  And since all the planning for this was done around a year ago it seemed like it was time to start.

I had wanted to see if toroids would offer a significant advantage over the tuned inductors in the RF bandpass filter.  While the existing filter works really well,  I considered moving from adjustable inductors, with a Q of perhaps 70, to toroids with a Q of say 200.

The tighter filter passband became less attractive when I worked out the maximum inductance I could wind on the toroids was around 1uH. A larger inductor has wire too fine to allow for compression and expansion of the coil turns to tune the filter. The advantages of the higher Q are offset by the lower impedances within the filter and the net result is not much difference. Had I had a larger toroid then things would have been different. So I will stick with the existing filter and this iteration will really be about incorporating all the modifications I made on the fly.

Here is a piece of advice I agonised over putting to print. I'm not comfortable denigrating other people's designs. But I was gob-smacked to see someone recommend an AM broadcast filter for a 40m receiver as a way of fixing receiver overload from short-wave transmitters. I'd like to know how that works. Because I can see no way that it can work. Remember, AM broadcast signals are around 1MHz. Those pesky short-wave signals run from 40m upwards to perhaps 16MHz.

And another matter that galls me is people still promoting simplistic designs as being worthwhile. If the project doesn't have at least a double tuned band pass filter between the aerial and first mixer then it's a glorified crystal set. I do wonder if those that propose the "easy" way know what they are doing.  For all the effort that goes into building a receiver reward yourself many times over with another 1% of effort and put in two or three extra components in one of the most critical parts of your receiver.

Here's a picture of progress to date. I hope to have this finished by the end of the month. Then I'll think about what to do with all the spare boards. Any takers??

Richard VK6TT

Monday, 12 February 2018

EM 10.5-10.68GHz microwave modules - Mods to use on 10.368GHz - update

The first RX module which down converts 10GHz to 144MHZ was finally finished today. Initial results were disappointing. However, after some modifications to improve the noise characteristics of the first local oscillator everything is running nicely now.

The conversion takes a while but the result is a sensitive down converter suitable for SSB and FM use. I'm not posting full details of all the changes required at this point. Please get in touch if you'd like to know more and I'll consider blogging further details.

In the meantime I have a few more modules to convert. The RX module has the following model identifiers:

em                   FM RECEIVER
10.5-10.68 GHz, Synth
P/N 036D                    S/N 111xxx

Wednesday, 17 January 2018

EM 10.5-10.68GHz microwave modules - Mods to use on 10.368GHz

This started out as what appeared to be an ambitious project but it is so far straightforward. I was fortunate enough to be given the chance to modify a few sets of the FM Tx and Rx boxes for ham use. It transpires that just about every set of modules is the same but different! They are all broadly similar but subtle variations exist so if you have some of these modules and yours don't match exactly then this shouldn't deter you.

Here's what I have done so far:
  1. Used a cheap logic analyser to check what was being loaded into the PLL chip by the micro. 
  2. Inserted my own micro to control the 2.5GHz PLL
  3. Achieved lock on several TX and RX units
PLL chip
The units I modified had the LM2326 PLL chip. It has a different data format than the LM2325 chip I saw on the units I have not yet modified.  In a few words the 2.5GHz PLL uses a 250KHz reference frequency. It is multiplied by 4 on both tx and rx units. On the tx units it is mixed with the IF, presently 69MHz but I am modifying this to use a 144MHz IF on tx. On Rx the multiplied signal is mixed with a 692MHz oscillator down to a 140MHz IF. At present I'm leaving this oscillator alone since the saw filter appears wide enough to be serviceable with a 2m IF.

Once I knew what the existing data stream was it was relatively simple to work out the data stream needed to load the PLL chip with data to put it on the desired frequency. See the chart below for an overview. I used an ATTINY13 and a few lines of assembly to do this.

Achieving lock.
This should be straightforward but sometimes the soldering proves difficult due to the heatsinking the board has. I moved the short until the PLL locked with around 4V-6V on the tuning voltage link.

I'll post some more pics soon but for now the following may prove useful.

An unmodified receiver showing what is required to modify for 10.368GHz use
Details of how the IF signal on Tx gets to the mixer diode for up-conversion to 10GHz
Data loaded into PLL chips to shift to 10.368GHz

Thursday, 5 October 2017

Lead Acid Batteries - Measurement of Internal Resistance - Results

It's been about 4 weeks now and some observations of the impact pulse conditioning has had on internal resistance and perhaps battery capacity are warranted. More importantly, the need for a more rigorous approach has become apparent.

Over that last 4 weeks the measured internal resistance has fallen from 264 milli-ohms to 175 milli-ohms. While the reduction in the internal resistance tapered off after about 10 days, there is some support for the battery's capacity having increased as well. At the start I was recharging the battery after 3 days on the pulse conditioner. Now it runs for 5 to 6 days before I have to recharge the battery.

So my initial observations are that the pulse conditioner is beneficial. However, there are several issues with the approach and the results cannot be construed as anything other than weak support for pulse conditioners.

The biggest drawback to the approach is the lack of a control battery. With a second battery I could cycle one on the pulse conditioner whilst the other was cycled on a static load. If the static load battery showed little, or no improvement, in internal resistance then there would be much stronger support for the pulse conditioning approach.

Another drawback stems from the lack of automation. I still have to manually changeover the battery at each step of the pulse, charge, measure cycle. So the time between measurements is not constant and the level of charge and discharge varies from one cycle to the next. I clearly need to automate the cycle and let it run unattended.

The final obvious shortcoming is temperature. We are moving towards summer and the average ambient temperatures have increased over the last month. The temperature change could be influencing the results either directly via battery chemistry somehow, or indirectly by it's impact on the voltage regulator which serves at the voltage reference for the D2A conversion. A temperature controlled testing environment would be useful to remove another source of potential error but that is beyond my reach at present.

The biggest obstacle to the full automation with the W1209 board is the need for an additional two digital outputs. I have considered two approaches. The first is using the + and - keys as both inputs and outputs. That would require some careful soldering to insert a resistor, say 2k, in series with each switch. The second approach is a 4017 counter clocked by the pin driving the relay. Then each of the decoded outputs for 1-3 from the 4017 driving a relay to perform each step of the pulse charge measure cycle.

I'm leaning towards the first approach since I don't know if I have a 4017 in the drawer and the first approach avoids any ambiguity over which step the cycle is in.

Tuesday, 12 September 2017

Lead Acid Batteries - Measurement of Internal Resistance - V3 of Code

​After hours running various tests to determine when the voltage samples should be taken I concluded there was no right answer. Mind you, the sweet joy of using Forth to do this via the serial port was a reward in itself. ( Timing loops could be tested interactively, results displayed on the terminal window, etc etc. )

I settled on a longer delay before sampling the loaded voltage, or Vend in the code, then a short delay before sampling the unloaded voltage, or Vbeg in the  code. Because of the way the voltage depresses over time, then recovers, I doubt there is a right way to do this. But my training in Statistical Process Control and Gauge Capability steered me in this direction.

I now have a testing method that gives me repeatable readings provided each test is run hours apart. So while the calculated internal resistance might not be the value according to some standard it should allow me to discern if my pulse conditioning circuit has any impact over time on the internal resistance.

Over the next few weeks I'll run my trials and see what happens.

Monday, 11 September 2017

Lead acid batteries - Measurement of internal resistance - x4 resolution mod

The initial project used a voltage divider to reduce the measured voltage down to less than 5V. This means the voltage resolution is about 20mV which means the internal resistance measurement for 1 amp of current is 0.020 ohms.

Instead of dividing by 4 with a resistor string, what if we subtracted 10V from the voltage to be measured? This would give us a voltage resolution of about 5mV with an internal resistance measurement for 1 amp of current is 0.005 ohms. This seems like a worthwhile improvement and can be easily achieved.

I had never played with a voltage subtracter before but it proved to work first time. Reaching into the junk box I pulled out a NE5532 dual op amp. I had no reason to chose this device over any other except I had hundreds I had recovered from a couple of boards. A few resistors and it was done.

While I used a NE5532 and 2.4k resistors I don't think there is anything critical about this. Just about any op amp will probably work and the resistors could be anything between perhaps 1k and 100k. As long as they are all the same value.

As before I'll post the updated code over on Hackaday. While this is not yet the complete measurement tool I wanted it will allow me to make some measurements on the impact, if any, pulse conditioning has on internal resistance.

Sunday, 13 August 2017

Lead acid batteries - Measurement of internal resistance - Code

While working on a small hardware modification to improve the resolution of measurements it occurred to me that displaying an internal resistance of 0.2 ohms as 200 was easier to read than 2. I updated the code over on  Hackaday

Debugging a change like this is so easy with Forth, especially when a simple x100 instruction fails because the 16 bit integer maths overflows.


ps the hardware mod worked a treat giving a fourfold improvement in resolution. I'll describe it shortly once I've finished the soldering. It might even mean that no modifications to the board are required!